The present invention relates to a front suspension device for an automotive vehicle, and particularly to a front suspension device for an automotive vehicle, in which wheel support members of right and left front wheels are coupled with each other via a steering unit, an outer end of each of two lower arms is pivotally attached to the wheel support members respectively, and an inner end of each of two lower arms is pivotally attached to a vehicle body via a resilient member.
In general, in a double wishbone type of front suspension device, an upper arm and a lower arm support a wheel via a wheel support member.
The lower arm of the double wishbone type of front suspension device is generally comprised of an A-shaped arm comprised of two arms formed integrally with each other, and the A-shaped arm is connected to the wheel support member via a single ball joint at an (outer) end thereof. This type of front suspension device is superior in the function of tire's contact with ground and the straight driving stability, but it has a disadvantage that it is difficult to set a kingpin axis at an ideal one.
Further, a double pivot type of front suspension device, a different type in the double wishbone type of front suspension device, is known which comprises a lower arm comprised of two independent I-shaped arms which are connected to the wheel support member via ball joints, respectively (for example, U.S. Pat. Nos. 4,863,188 and 5,348,337). This double pivot type of front suspension device can constitute a fictitious kingpin axis passing through an intersection of extended axes of the two I-shaped arms and therefore provide a positive offset of the kingpin properly, thereby improving a vehicle driving stability under a braking condition by controlling wheels so as to change in a displacement in toe-in direction.
In the two types of front suspension device described above, a damping device, which is comprised of a coil spring, a shock absorber and so on, is normally attached to the lower arm (see U.S. Pat. No. 4,863,188).
Meanwhile, the inventors of the present invention have researched eagerly the above-described double pivot type of front suspension device and found out possibilities of preventing the straight driving stability of vehicle from deteriorating and improving the maneuverability and stability by solving a problem caused by the coil spring of the damping device attached which is to the lower arm.
Namely, the coil spring has a characteristic that when it is contracted or extended from its free state, both ends of the coil are apt to rotate about a coil axis in an opposite direction to each other. The inventors, paying their attentions to this characteristic, have figured out the following. Namely, the coil spring of the damping device under a normal driving condition is kept in a contracted state by receiving a vehicle weight. At this time, there occurs a rotational torque at the coil spring because of a winding of the coil spring, and the rotational torque is conveyed to the lower arm. Accordingly, a bush (resilient member), which is provided at an attaching portion of the lower arm to a vehicle body, is deformed by the conveyed rotational torque. As a result, the lower arm changes in a displacement and then a toe angle of the wheel also changes, resulting in deterioration of the straight driving stability of vehicle. Also, a caster trail of the wheel changes, resulting in deterioration of the straight driving stability of vehicle as well.
Herein, the above-described characteristics of the coil spring will be explained more referring to FIG. 12. FIG. 12 is a conceptual diagram for explaining a force acting on the coil spring and a force occurring at the same.
As shown in FIG. 12, when a compressing force P is added to a coil spring 100 with a counterclockwise-winding direction, when viewed from above, in a coil axis direction, there occurs a rotational torque E which is apt to rotate an upper end and an lower end of the coil spring 100 in an opposite direction to each other in such manner that the upper end rotates counterclockwise about the coil axis and the lower end rotates clockwise about the coil axis. As a result, if both of the upper and lower ends of the coil spring are fixed, the rotational torque occurs which are apt to rotate the upper end counterclockwise and the lower end clockwise.
The inventors of the present invention also have found out how the lower arm may change in a displacement according to this characteristic of the coil spring and how the displacement change of the lower arm may influence changes of toe angle and caster trail of the wheel (front wheel), which will be explained referring to FIGS. 13 to 15.
FIG. 13 is a diagram for showing schematically a positional relationship between the lower arm and the front wheel under “a full rebound condition” where a right front wheel of a front suspension device does not contact on the ground and a vehicle weight does not act on a damping device; an elevation view FIG. 13(a) and a plan view FIG. 13(b). Meanwhile, FIG. 14 is a diagram for schematically showing a positional relationship between the lower arm and the front wheel under a vehicle stop condition or a straight driving condition with a constant speed (hereinafter, referred to as “1 G vehicle height condition”) where the right front wheel of the front suspension device contacts on the ground and the vehicle weight acts on the damping device; an elevation view FIG. 14(a) and a plan view FIG. 14(b). Further, FIG. 15 is a plan view for schematically showing a positional relationship between the lower arm and the left front wheel under the 1 G vehicle height condition.
In the front suspension device shown in FIGS. 13 to 15, a winding direction of the coil spring 104 of the damping device 102 is a counterclockwise one for both of the right and left front wheels.
Although the coil spring 104 is extended to a certain degree receiving (supporting) weights of the wheel and the like under the full rebound condition shown in FIG. 13, the lower arms 106, 108 and the right front wheel 110a are located at their substantially regular positions.
Meanwhile, as shown in FIG. 14, the coil spring 104 is contracted under the 1 G vehicle height condition (vehicle stop condition or straight driving condition with constant speed). At this moment, since the coil spring 104 has the counterclockwise-winding direction, there occurs a rotational torque F with clockwise direction at the lower end of the coil spring 104. The lower arm 106 receives this rotational torque, and then bushes 112, 114 provided at attaching portions of the lower arms 106, 108 to the vehicle body are deformed. As a result, the lower arms 106, 108 and the right front wheel 110a change in displacement such that a forward end of the right front wheel moves toward inside and eventually the wheel changes in a toe angle, as shown in FIG. 14.
In general, the coil springs of the damping devices of both of the right and left wheels have the same winding direction (in this case, the counterclockwise direction) for the purpose of a commonality of parts. Accordingly, the direction of the rotational torque conveyed to each lower arm is asymmetric with respect to the right and left (in this case, the same clockwise direction), and the left front wheel changes in a displacement such that a forward end of the left front wheel 110b moves toward outside, as shown in FIG. 15. As a result, both of the right and left front wheels change in their toe angles so as to point in the same direction, resulting in deterioration of the straight driving stability of vehicle.
Further, the displacement of the lower arm also changes a caster trail of the wheel. If the displacement of the lower arm is asymmetric with respect to the right and left wheels as shown in FIGS. 14, 15, there occurs a difference of caster trail between the right and left wheels. The difference of the caster trail may bring about a difference of self aligning torque between the right and left wheels, thereby deteriorating the straight driving stability of vehicle. Particularly, when being influenced by an irregular load surface such as a rut and a cant under a straight driving condition, the difference of self aligning torque may influence inappropriately the straight driving stability of vehicle.
Herein, although there occurs a similar displacement of the lower arm in the above-described front suspension device with A-shaped arm because of deformation of bush, the arm of this type is comprised of two arms formed integrally with each other. Accordingly, it was also figured out by the inventors that there hardly occurs any displacement of the lower arm to cause such toe-angle change of the wheel and influence inappropriately the straight driving stability in this type of the front suspension device.